Enhancing the Effectiveness of Oxygen Evolution Reaction by Electrodeposition of Transition Metal Nanoparticles on Nickel Foam Material

catalysts

Article Enhancing the Effectiveness of Oxygen Evolution Reaction by Electrodeposition of Transition Metal Nanoparticles on Nickel Foam Material

Mateusz Łuba 1 , Tomasz Mikołajczyk 1,* , Mateusz Kuczy ´nski 1, Bogusław Pierozy˙ ´nski 1,* and Ireneusz M. Kowalski 2

1 Department of Chemistry, Faculty of Agriculture and Forestry, University of Warmia and Mazury in Olsztyn, Plac Lodzki 4, 10-727 Olsztyn, Poland; [email protected] (M.Ł.); [email protected] (M.K.) 2 Department of Rehabilitation, Faculty of Medical Sciences, University of Warmia and Mazury in Olsztyn, Zolnierska 14C Street, 10-561 Olsztyn, Poland; [email protected] * Correspondence: [email protected] (T.M.); [email protected] (B.P.); Tel.: +48-89-523-4177 (B.P.)

Abstract: Electrochemical oxygen evolution reaction (OER) activity was studied on nickel foam-based electrodes. The OER was investigated in 0.1 M NaOH solution at room temperature on as-received and Co- or Mo-modiﬁed Ni foam anodes. Corresponding values of charge-transfer resistance, exchange current-density for the OER and other electrochemical parameters for the examined Ni

foam composites were recorded. The electrodeposition of Co or Mo on Ni foam base-materials resulted in a signiﬁcant enhancement of the OER electrocatalytic activity. The quality and extent

Citation: Łuba, M.; Mikołajczyk, T.; of Co, and Mo electrodeposition on Ni foam were characterized by means of scanning electron Kuczyński,M.; Pierozyński,B.;˙ microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) analysis. Kowalski, I.M. Enhancing the Effectiveness of Oxygen Evolution Keywords: oxygen evolution reaction; Ni foam; Co-modified Ni foam; Mo-modified Ni foam; Reaction by Electrodeposition of electrodeposition; electrochemical impedance spectroscopy Transition Metal Nanoparticles on Nickel Foam Material. Catalysts 2021, 11, 468. https://doi.org/10.3390/ catal11040468 1. Introduction In recent years, the development of renewable energy sources has dramatically in- Academic Editor: Yongjun Feng creased [1,2], resulting from both a gradual depletion of crude oil sources, as well as progressive degradation of the natural environment [3,4]. Unfortunately, currently-used Received: 14 March 2021 green energy technologies are strongly dependent on weather conditions and topography. Accepted: 1 April 2021 Published: 3 April 2021 For instance, the energy production of solar panels reaches its peak value at noon; however, it does not meet the demands of energy consumption [5]. This phenomenon is well-known

Publisher’s Note: MDPI stays neutral under the name of solar energy’s duck curve. Appropriate storage of this energy excess with regard to jurisdictional claims in could be the only solution to the above problem [6]. Currently, most of the produced published maps and institutional afﬁl- renewable energy is stored in lithium-ion battery systems, because of their high-efﬁciency iations. and convenience in exploitation. However, all batteries have their limitations, such as slow charge and discharge time, which is strongly dependent on ambient temperature, as well as limited operational lifetime. Moreover, used batteries could become environmentally important hazardous waste. One of the most promising new energy storage technologies is based on hydrogen generation by means of alkaline water electrolysis (AWE) [7,8]. Such Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. produced hydrogen can be considered an entirely environmentally friendly energy carrier, This article is an open access article since the only by-products generated during the hydrogen combustion are heat and water distributed under the terms and (see Equation (1)) [9]. conditions of the Creative Commons 2H2 + O2 → 2H2O + Q (1) Attribution (CC BY) license (https:// While many catalysts are known to reduce the overpotential of hydrogen evolution creativecommons.org/licenses/by/ reaction (HER), the more complex oxygen evolution (OER) process still requires a large 4.0/).

Catalysts 2021, 11, 468. https://doi.org/10.3390/catal11040468 https://www.mdpi.com/journal/catalysts Catalysts 2021, 11, 468 2 of 13

amount of energy to proceed. Contrary to a comparatively fast single-electron transfer cathodic process, the OER is related to a slow and complex, four-electron transfer reaction (see Equation (2)) [10,11]. − − 4OH → O2 + 2H2O + 4e (2) Hence, in order to minimize the overpotential of the OER, the researchers have made significant efforts in developing new types of anodes. As a result, electrodes based on ruthenium [12] and iridium [13] oxides were found to exhibit outstanding electrochemical properties towards the OER; however, because of their high costs, the use of these materials on the industrial scale is not economically practical. Therefore, in recent years, the researchers’ attention has been shifted to more abundant and inexpensive transitional metals (Ni, Co, Mn, Mo, Fe) [14–16] and their alloys [17], oxides and carbides [18,19] to replace platinum group metals. Thus, one of the most commonly used base materials to make electrodes, in the hydrogen production industry, are nickel and its derivatives. The employment of nickel and nickel-coated electrodes results from their unique properties, such as high catalytic activity in both hydrogen and oxygen evolution reactions. In addition, nickel exhibits superior anticorrosion properties in an alkaline environment. Generally, it is well-known that the OER on metals proceeds on the surface covered by their oxides, where nickel oxide/hydroxide species are especially catalytic in this reaction [20–26]. However, in order to compete with the electrocatalytic properties of electrodes based on noble metals, newly developed electrodes have to be characterized by large and highly active surface areas (HASA). Large values of HASA could be easily achieved by employing various materials, including foams, fibres, nanowires or nanocones. Such a base material could then be modified with a trace amount of transition metal(s) by one of the deposition methods (electrochemical, electroless, CVD: chemical vapour deposition or PVD: physical vapour deposition), which could further enhance the electrocatalytic properties of these innovative materials [20,21]. Nickel foam could be considered one of the most promising candidates for fabricating such a highly reactive anode in the process of the OER, as in addition to its unique electrocatalytic properties, Ni foam also provides large values of HASA. Nickel is also relatively inexpensive, compared to most noble metals [22]. In this work, electrochemically modified Ni foam materials were assessed as potential anodes towards the OER in alkaline 0.1 M NaOH environment through the employment of major electrochemical techniques. In addition, the structure and quality of electrodeposited Co and Mo on the surface of nickel foam were evaluated by means of a combined scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) methodology.

2. Results and Discussion 2.1. SEM/EDX Characterization of Ni Foam and Co-, and Mo-Modified Nickel Foam Electrodes Figure1a,b below illustrate SEM micrograph pictures of the electrodeposition effect of Co and Mo at small and trace amounts, respectively, on the Ni foam surface, recorded at the same magnification level. For the Co-modified Ni foam (at ca. 1 wt.% Co), the surface of the base material is densely covered with a granular structure of small Co nuclei (see Figure1a). On the other hand, a sample of Mo-modified Ni foam (at ca. 0.2 wt.% Mo) is comparatively shown in a SEM micrograph picture of Figure1b. The electrodeposited sample is covered with quite an irregular, amorphous and discontinuous structure through the Ni foam surface. In addition, the more detailed information on the composition of the prepared samples is shown in Table1. Moreover, Figure1c,d demonstrate the EDX pattern of Co- and Mo-modified Ni foam electrodes, respectively. In addition, the SEM-approximated Co and Mo grain size value came to 50.0 ± 3.5 and 512 ± 9.8 nm, respectively. The above was performed through utilization of the Image Analysis Program (NIS-Elements Basic Research on Nikon). Catalysts 2021, 11, x FOR PEER REVIEW 3 of 13

Catalysts 2021, 11, 468 3 of 13 shown in Table 1. Moreover, Figure 1c,d demonstrate the EDX pattern of Co- and Mo- modified Ni foam electrodes, respectively.

Figure 1. SEM micrograph pictures of Co-modiﬁed Ni foam surface (a), and Mo-activated Ni foam surface (b), taken at 5000× magniﬁcationFigure 1. withSEM EDXmicrograph pattern ofpictures Co- (c )of and Co-modified Mo-activated Ni foam (d) Ni surface foam samples. (a), and Mo-activated Ni foam surface (b), taken at × 5000 magnification with EDX pattern of Co- (c) and Mo-activated (d) Ni foam samples.

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Table 1. EDX-derived (for an acceleration voltage of 15 kV) chemical composition of surface elements for freshly prepared: Co- and Mo-modified Ni foam samples. Catalysts 2021, 11, 468 4 of 13 Element/% Spectrum 1 Spectrum 2 Co-modified Ni foam Ni 95.83 92.84 Co 1.06 1.25 Table 1. EDX-derived (for an acceleration voltage of 15 kV) chemical composition of surface elements O 3.11 5.91 for freshly prepared: Co- and Mo-modified Ni foam samples. Total: 100.00 100.00 Mo-modified Ni foam Element/% Ni Spectrum 92.08 1 Spectrum 86.42 2 Co-modified Ni foam NiMo 0.25 95.83 0.33 92.84 CoO 2.61 1.06 5.06 1.25 OC 5.06 3.11 8.19 5.91 Total: 100.00 100.00 Total: 100.00 100.00 Mo-modified Ni foam Ni 92.08 86.42 Mo 0.25 0.33 In addition, the SEM-approximated Co and Mo grain size value came to 50.0 ± 3.5 and O 2.61 5.06 512 ± 9.8 nm, respectively. The Cabove was performed 5.06 through utilization 8.19of the Image Analysis Program (NIS-ElementsTotal: Basic Research on 100.00Nikon). 100.00

2.2.2.2. OEROER CharacterizationsCharacterizations ofof NiNi Foam,Foam, Co-Co- andand Mo-ModifiedMo-Modified NiNiFoam FoamElectrodes Electrodes CyclicCyclic voltammetryvoltammetry (CV) (CV) characterization characterization of of the the surface surface oxidation oxidation process process on fresh, on fresh, Co- Co-and andMo-modified Mo-modified Ni foam Ni foam electrodes electrodes in 0.1 inM 0.1NaOH M NaOH solution solution during during50 CV sweeps, 50 CV sweeps, carried- carried-outout over the over potential the potential span 1.2–1.9 span V/RHE 1.2–1.9 (rever V/RHEsible (reversible hydrogen hydrogen electrode) electrode) with a scan with rate aof scan 50 mV rate s−1 of (only 50 mV three s− last1 (only sweeps three are last presented) sweeps areis shown presented) in Figure is shown 2 below. in FigureThus, for2 below.pure and Thus, Co-modified for pure and nickel Co-modified foam samples, nickel the foam CV profiles samples, displayed the CV profiles an oxidation displayed peak an(centred oxidation at ca. peak 1.49 (centred and 1.45 at ca.V, 1.49correspondingly) and 1.45 V, correspondingly) along with a cathodic along with reduction a cathodic peak reduction(starting around peak (starting 1.3 V). aroundThe Mo 1.3 modification V). The Mo ca modificationused a shift causedof the oxidation a shift of thepeak oxidation to more peakpositive to more potentials positive than potentials that for pure than thatNi foam. for pure In contrast, Ni foam. the In reduction contrast, thepeak reduction was no longer peak wasvisible, no longeras it was visible, most likely as it wasmoved most outside likely the moved measured outside potential the measured range. These potential features range. refer Theseto the featureselectrochemical refer to formation the electrochemical of the surface formation β-NiOOH of the phase surface and itsβ-NiOOH reduction phase to β-Ni(OH) and its 2 reductionlayer, correspondin to β-Ni(OH)gly 2(Equationlayer, correspondingly (3)). (Equation (3)).

β-Ni(OH)2 + OH¯ ↔ β-NiOOH + H2O +− e− (3) β-Ni(OH)2 + OH¯ ↔ β-NiOOH + H2O + e (3)

FigureFigure 2.2. Cyclic voltammogram voltammogram curves curves of ofoxidized, oxidized, pure pure Ni, Ni,Co- Co- and andMo-activated Mo-activated Ni foam Ni foamelectrodes, elec- ◦ −1 trodes,in contact in contactwith 0.1 with M NaOH 0.1 M medium NaOH medium (at 20 °C), (at carried-out 20 C), carried-out at a scan-rate at a of scan-rate 50 mV s of−1 for 50 the mV potential s for thespan potential 1.2–1.9 spanV vs. 1.2–1.9RHE. V vs. RHE. Tafel polarization plots recorded at room temperature for electrochemically oxidized, pure Ni foam, Mo- and Co-modiﬁed Ni foam electrodes are shown in Figure3 . Hence, for the nickel foam electrode, the recorded anodic Tafel slope (parameter ba in Figure3 ) and −1 the Tafel-derived exchange current-density (j0) value for the OER came to 54 mV dec and 7.1 × 10−11 A cm−2 over the low overpotential range. The modiﬁcation utilizing Catalysts 2021, 11, x FOR PEER REVIEW 5 of 13

Catalysts 2021, 11, 468 Tafel polarization plots recorded at room temperature for electrochemically oxidized,5 of 13 pure Ni foam, Mo- and Co-modified Ni foam electrodes are shown in Figure 3. Hence, for the nickel foam electrode, the recorded anodic Tafel slope (parameter ba in Figure 3) and the Tafel- derived exchange current-density (j0) value for the OER came to 54 mV dec−1 and 7.1 × 10−11 A surfacecm−2 over Co the or low Mo overpotential deposition caused range. theThe values modification of ba and utilizingj0 parameters surface Co to or rise Moto deposition 83 and −1 −8 −8 −2 74caused mV dec the values, and 7.0of b×a and10 j0 andparameters 9.6 × 10 to riseA cmto 83 ,and correspondingly, 74 mV dec−1, and for 7.0 the × Co- 10−8 and and Mo- 9.6 × modiﬁed10−8 A cm electrodes.−2, correspondingly, Simultaneously, for the forCo-larger and Mo-modified overpotential electrodes. values, the Simultaneously, reaction system for (notlarger presented overpotential in Figure values,3 ) moved the reaction into the system mass (not transport presented limitations in Figure (see 3) e.g.,moved Ref. into [27 ]).the Pre-electrooxidationmass transport limitations of Ni foam(see e.g., surfaces Ref. [27]). proved Pre-electrooxidation to be necessary to of increase Ni foam an surfaces initial proved value ofto the be jnecessary0 parameter to increase for low an overpotential initial value of range, the j0as parameter compared for to low an overpotential analogous Ni range, foam- as basedcompared electrode to an byanalogous 3.6× [23 Ni]. Additionally,foam- based electr such-recordedode by 3.6 × exchange[23]. Additionally, current-density such-recorded was higherexchange than current-density that for other Niwas foam higher [24 ]than and that similar for 3D-structuredother Ni foam [ 25[24]] Ni and electrodes similar by3D- 7.0structured× and 1.6 [25]×, respectively.Ni electrodes Moreover,by 7.0 × and as 1.6 compared ×, respectively. to similar Moreover, catalysts as compared [25], Mo-modiﬁed to similar Nicatalysts foam electrode [25], Mo-modified exhibited superior Ni foam OER electro catalyticde exhibited activity (withsuperior over OER 17× catalytichigher value activity of the j parameter) to that of Ni-Mo composite coating. (with0 over 17 × higher value of the j0 parameter) to that of Ni-Mo composite coating.

−1 FigureFigure 3. 3.Quasi Quasi-potentiostatic-potentiostatic anodic anodic Tafel Tafel polarization polarization curves curves (recorded (recorded at at a scan-ratea scan-rate of of 0.5 0.5 mV mV s− s1) ) forfor oxidized oxidized Ni Ni foam, foam, Co- Co- and and Mo-activated Mo-activated Ni Ni foam foam electrodes, electrodes, carried carried out out in in 0.1 0.1 M M NaOH NaOH solution solution (appropriate iR corrections were made based on the solution resistance derived from impedance (appropriate iR corrections were made based on the solution resistance derived from impedance measurements); calculated Tafel slopes for oxidized Ni foam, Co- and Mo-modified Ni foam measurements); calculated Tafel slopes for oxidized Ni foam, Co- and Mo-modified Ni foam samples samples came to ba = 54, 83 and 74 mV dec−1, correspondingly. −1 came to ba = 54, 83 and 74 mV dec , correspondingly. Nowadays, there is a trend in literature to show the Tafel polarization results for a Nowadays, there is a trend in literature to show the Tafel polarization results for a set set overpotential, typically at η = 0.300 V or a set current-density, usually at 10 mA cm−2 [26,27]. overpotential, typically at η = 0.300 V or a set current-density, usually at 10 mA cm−2 [26,27]. Hence, the current densities at η = 0.3 V for the Co-modified Ni foam electrode came to 9.2 × Hence, the current densities at η = 0.3 V for the Co-modified Ni foam electrode came to 10−5 A cm−2, which was superior to those recorded in literature for various cobalt-modified 9.2 × 10−5 A cm−2, which was superior to those recorded in literature for various cobalt- −6 −5 −2 modifiedanodes (see anodes e.g., (see the e.g., recorded the recorded values values of 1.4 of× 1.410 × –10 1.0−6 –1.0× 10× A10 −cm5 A cmfor− cobalt2 for cobalt oxide electrodes by Lyons and Brandon in Ref. [28]. On the other hand, the recorded ba oxide electrodes by Lyons and Brandon in Ref. [28]. On the other hand, the recorded ba parameterparameter for for the the examined examined electrodes electrodes was similarwas similar to those to derived those derived on Co and on Mo Co modified and Mo −1 3D-structuredmodified 3D-structured electrodes (39electrodes to 80 mV (39 dec to−1 )80 by mV Kubisztal dec ) andby Budniok,Kubisztal Badruzzamanand Budniok, etBadruzzaman al. and Lyons, et and al. and Brandon Lyons, in and References Brandon [25 in– 27References]. [25–27]. InIn order order to to furtherfurther characterize characterize the the electrocatalytic electrocatalytic properties properties of Ni of foam-based Ni foam-based electrodes elec- trodestowards towards the OER, the the OER, electrochemical the electrochemical impedanc impedancee spectroscopy spectroscopy (EIS) technique (EIS) was technique applied. wasThe applied. complex-plane The complex-plane EIS spectra are EIS shown spectra in are Figure shown 4. inIt should Figure4 be. It mentioned should be that men- all tionedelectrodes that have all electrodes exhibited havetwo somewhat exhibited depre two somewhatssed semicircles, depressed where semicircles, the high-frequency where the arc high-frequencycould be associated arc could with bethe associated kinetics of with the theoxygen kinetics evolution of the oxygenreaction evolution (interfacial reaction charge- (interfacial charge-transfer reaction step). In contrast, the intermediate/low-frequency semicircle corresponds to the surface adsorption of the reaction intermediates [29–34]. The electrochemical parameters [Faradic reaction resistance (Rct), the adsorption resistance CatalystsCatalysts 2021 2021, 11, ,11 x ,FOR x FOR PEER PEER REVIEW REVIEW 6 of6 of13 13

Catalysts 2021, 11, 468 6 of 13 transfertransfer reaction reaction step). step). In In contrast, contrast, the the intermediate/low-frequency intermediate/low-frequency semicircle semicircle corresponds corresponds to to thethe surface surface adsorption adsorption of ofthe the reaction reaction interm intermediatesediates [29–34]. [29–34]. The The electrochemical electrochemical parameters parameters [Faradic[Faradic reaction reaction resistance resistance (R (ctR),ct ),the the adsorption adsorption resistance resistance for for reaction reaction intermediates intermediates (R (AdsRAds), ), for reaction intermediates (R ), double-layer capacitance (C ) and pseudo-capacitance double-layerdouble-layer capacitance capacitance (C (dlC) dland) Adsand pseudo-capacitance pseudo-capacitance (C (AdsCAds)] )]were weredl obtained obtained by by means means of oftwo two C constant(constantAds)] phase were phase element obtained element (CPE)-modified by(CPE)-modified means of two Randles Randles constant equivalent equivalent phase circuit element circuit model model (CPE)-modiﬁed (Figure (Figure 5), 5), and and Randles their their equivalent circuit model (Figure5), and their values are presented in Table2. The CPE valuesvalues are are presented presented in in Table Table 2. 2.The The CPE CPE elem elementent was was used used within within the the circuit circuit in in order order to to element was used within the circuit in order to account for the capacitance dispersion effect, accountaccount for for the the capacitance capacitance dispersion dispersion effect, effect, represented represented by by distorted distorted semicircles semicircles in in the the represented by distorted semicircles in the Nyquist impedance plots. NyquistNyquist impedance impedance plots. plots.

FigureFigure 4. 4.Electrochemical ElectrochemicalElectrochemical impedance impedance impedance Nyquist Nyquist Nyquist plots plots plots for for the for the OER the OER OERon on oxidized oxidized on oxidized Ni Ni foam, foam, Ni foam,Co- Co- and and Co- Mo- Mo- and ◦ modifiedMo-modiﬁedmodified Ni Ni foam foam Ni electrode foam electrode electrode surfaces surfaces surfaces in incontact contact in contact wi with th0.1 with 0.1 M M NaOH 0.1 NaOH M NaOH (at (at 20 20 °C) (at °C) 20for for theC) thefor potential potential the potential of of1.6 1.6 of V 1.6Vvs. vs. VRHE. vs.RHE. RHE.The The solid The solid solidline line corresponds line corresponds corresponds to torepresentati torepresentati representationonon of ofthe ofthe data the data data according according according to toequivalent to equivalent equivalent circuit circuit circuit presentedpresented in inFigure Figure 5 5 below.5 below. below.

Figure 5. Equivalent circuit for impedance analysis of the OER process. Two CPE-R element Figure 5. Equivalent circuit for impedance analysis of the OER process. Two CPE-R element equivalent Figureequivalent 5. Equivalent circuit circuit was used for impedance for ﬁtting analysis the impedance of the OER data process. for as Two received CPE-R Ni element foam, Co-equivalent and Mo- circuitcircuit was was used used for for fitting fitting the the impedance impedance data data for for as asreceived received Ni Ni foam, foam, Co- Co- and and Mo-activated Mo-activated Ni Ni foam foam activated Ni foam electrodes, obtained in 0.1 M NaOH solution. The circuit includes two constant electrodes,electrodes, obtained obtained in in0.1 0.1 M MNaOH NaOH solution. solution. The The circuit circuit includes includes two two constant constant phase phase elements elements (CPEs), (CPEs), phase elements (CPEs), to account for distributed capacitance; Rct, Cdl, RAds, and CAds (as CPE) to toaccount account for for distributed distributed capacitance; capacitance; R ctR, ctC, dlC, dlR, AdsRAds, and, and C AdsCAds (as (as CPE) CPE) elements elements correspond correspond to tothe the charge-transferelementscharge-transfer correspond resistance, resistance, to theinterfac interfac charge-transferialial double-layer double-layer resistance, capacitance, capacitance, interfacial resi resistance double-layerstance and and capacitance capacitance capacitance, adsorption adsorption resistance componentsandcomponents capacitance of ofreaction reaction adsorption intermediates; intermediates; components Rs Riss solutionis of solution reaction resistance. resistance. intermediates; Rs is solution resistance.

Thus, for the oxidized Ni foam electrode, the recorded Rct parameter diminished from

0.187 Ω g to 0.047 Ω g at η = 270 and 620 mV, respectively. Similarly, the Cdl parameter decreased from the value of 120,933 to reach 8720 µF g−1 sϕ1−1 for the same overpotential span (see Table2 for details). The above effect could be associated with substantial blocking of the electrode surface by freshly formed O2 bubbles, which could easily be visually

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noticed, especially at increased anodic overpotentials. Simultaneously, the value of the RAds parameter reduced from 41.756 to 0.123 Ω g for the corresponding potential range, while the CAds parameter oscillated between an initial value of 144,293 (at 270 mV) and a minimum value of 67,867 µF g−1 sϕ2−1 recorded at 420 mV. The latter understandably implies that the kinetics of the OER become facilitated upon the rise of the dc electrode potential. However, the RAds parameter values shown in Table2 are essentially larger than those recorded for the Rct parameter. The above suggests that the OER process is limited by the reaction intermediates’ adsorption kinetics (also see References [23,34]).

Table 2. Electrochemical parameters for the OER, obtained at as received Ni foam, Co- and Mo-modified Ni foam electrodes in contact with 0.1 M NaOH supporting solution. The results obtained here were recorded by fitting the CPE-modified Randles equivalent circuit (see Figure5) to the experimentally obtained impedance data (reproducibility usually below 10%, χ2 = 4.03 × 10−5 to 1.07 × 10−3.

−1 ϕ1−1 −1 ϕ2−1 η/mV Rct/Ω g Cdl/µF g s RAds/Ω g CAds/µF g s Ni Foam 270 0.187 ± 0.033 120,933 ± 12,554 41.756 ± 4.524 144,293 ± 11,985 320 0.129 ± 0.025 127,147 ± 12,430 9.047 ± 0.206 103,520 ± 11,383 370 0.073 ± 0.008 114,453 ± 6488 1.844 ± 0.105 81,360 ± 5257 420 0.086 ± 0.008 89,627 ± 3612 0.616 ± 0.008 67,867 ± 2587 470 0.075 ± 0.007 81,493 ± 3352 0.289 ± 0.007 73,840 ± 3282 520 0.061 ± 0.005 67,600 ± 3119 0.176 ± 0.005 90,933 ± 3627 570 0.064 ± 0.005 76,773 ± 2646 0.114 ± 0.005 78,717 ± 5113 620 0.047 ± 0.002 8720 ± 414 0.123 ± 0.003 129,227 ± 9950 Co-modiﬁed Ni foam 270 0.052 ± 0.006 541,047 ± 15,371 5.265 ± 0.417 670,980 ± 28,526 320 0.035 ± 0.004 224,899 ± 26,583 1.239 ± 0.009 690,541 ± 6229 370 0.030 ± 0.004 218,716 ± 10,076 0.217 ± 0.004 412,736 ± 7235 420 0.027 ± 0.003 188,682 ± 8730 0.214 ± 0.004 444,831 ± 6886 470 0.028 ± 0.003 141,993 ± 5113 0.128 ± 0.003 475,169 ± 10,406 520 0.024 ± 0.002 121,588 ± 3725 0.089 ± 0.002 488,446 ± 13,374 570 0.024 ± 0.002 161,622 ± 5287 0.052 ± 0.002 576,419 ± 31,772 620 0.022 ± 0.004 105,473 ± 9599 0.041 ± 0.005 557,230 ± 91,804 Mo-modiﬁed Ni foam 270 0.081 ± 0.011 822,987 ± 85,015 2.274 ± 0.052 1,743,525 ± 143,562 320 0.069 ± 0.010 540,724 ± 66,936 0.667 ± 0.018 1,916,698 ± 61,430 370 0.066 ± 0.009 599,202 ± 157,063 0.296 ± 0.007 1,680,668 ± 168,453 420 0.068 ± 0.017 1,024,082 ± 152,342 0.181 ± 0.014 1,045,659 ± 157,476 470 0.052 ± 0.004 200,204 ± 20,927 0.150 ± 0.006 1,512,226 ± 94,060 520 0.035 ± 0.008 783,766 ± 74,144 0.111 ± 0.008 1,337,607 ± 118,659 570 0.030 ± 0.011 1,008,571 ± 140,070 0.091 ± 0.011 916,327 ± 126,682 620 0.023 ± 0.003 1,271,429 ± 181,585 0.087 ± 0.003 1,524,620 ± 324,842

Then, the Rct parameter for the Co and Mo-modified Ni foam electrodes ranged from 0.052 to 0.022 Ω g and from 0.081 to 0.023 Ω g for the overpotential range: 270–620 mV, respectively. The former implies a considerable reduction of the Rct parameter (as compared to those Rct values recorded for the oxidized Ni foam surface); namely, by 3.6× and 2.3× at 270 mV, and by 2.1× at 620 mV for the Co and Mo-modified catalyst materials, respectively (see again Table2 for details). Similarly, changing radius of the low-frequency semicircle (adsorption of reaction intermediates response) also resulted in a relatively constant decrease of the RAds parameter value by ca. 8× and 3× for the cobalt-based, and by 18× and 1.4× for the Mo-modified Ni foam electrodes for the overpotentials of 270 and 620 mV. Furthermore, deposition of catalytic nanostructures (see Figure1a,b) resulted in a considerable enhancement of the electrochemically accessible surface area. The latter could be easily observed in the cyclic voltammetry plot presented for the baseline and Catalysts 2021, 11, 468 8 of 13

the Co and Mo-modified Ni foam materials in Figure2. Consequently, significant surface modification for the Co- and Mo-modified Ni foams is demonstrated by considerably increased current-density within the corresponding CV profiles. −1 ϕ1−1 The reported values of the Cdl parameter at η = 270 mV came to 541,047 µF g s (Co-modified Ni foam) and 822,987 µF g−1 s ϕ1−1 for the Mo-based Ni foam catalyst material, which translates to ca. 4.5× and 6.8× augmentation of the HASA values for both modified electrodes, in contrast to the oxidized Ni foam anode. Once these surface coefficients are referred to the previously calculated ratios of the Rct parameter, it becomes evident that the cobalt-activated nickel foam anode exhibited superior OER performance. In addition, dimensionless ϕ1 and ϕ2 parameters (where ϕ determines the constant phase angle in the complex-plane plot and 0 ≤ ϕ ≤ 1) of the CPE circuits (see Figure5) oscillated between: 0.66–0.97 and 0.49–0.97, respectively. The impedance results obtained for the examined electrodes in this work were generally in line with those OER impedance studies presented by Si et al. [35] and by Lyons, and Brandon [28] on cobalt oxide electrodes, and by Choi et al. [36] and by Wu, and He [37] on Mo-based composites. Moreover, Table3 presents a comparison of the impedance-recorded Rct and j (polarization plot-estimated for η = 0.3 V) parameter values in this work (the charge-transfer resistance was derived through the Cdl-approximated surface area of the nickel foam electrodes [23,38,39]) with the results recorded on similar electrodes in Refer- ences [35–37]. It should be noted that the Rct results presented for Ni-Mo based electrodes in Refs. [36,37] were given as raw data, without providing any specific information on the electrodes’ surface area. Thus, in order to make a meaningful comparison, one could estimate the HASA values through dividing the recorded interfacial capacitance by a commonly used value of 20 mF cm−2 in literature for smooth and homogeneous surfaces (see Refs. [38,39]). Having done so, the surface-normalized Rct parameter would come 2 to 981 and 364 Ω cm for Ni-Fe-C-Mo and NiMoO4 electrode surfaces, correspondingly. These estimated resistance values are radically greater than those recorded in this work. In addition, the plot-derived value of the j parameter (1.15 × 10−5 A cm−2) for Co oxide/Fe electrode came very close to the current-densities recorded in this work.

Table 3. Electrochemical parameters for the OER performed at the surfaces of different catalyst materials, under alkaline conditions.

2 −2 η/mV Solution Catalysts Rct/Ω cm j(η = 0.3V)/A cm Ref. 270 0.1 M NaOH Co-modified Ni foam 31.9 9.2 × 10−5 This work 420 0.1 M NaOH Co-modified Ni foam 16.6 - This work 430 0.1 M KOH Co(OH)2/C 8.5 - 35 420 0.1 M NaOH Co oxide/Fe 17.5 1.15 × 10−5 28 270 0.1 M NaOH Mo-modified Ni foam 27.1 1.6 × 10−4 This work 420 0.1 M NaOH Mo-modified Ni foam 22.6 - This work 290 1.0 M KOH NiMoO4 nanowire/Ni foam 981 * - 36 210 30% KOH Ni-Fe-C-Mo/Ni mesh 364 * - 37 438 1.0 M NaOH Pt 80 ~4 × 10−5 ** 40 −4 278 0.1 M KOH RuO2/GC *** 142 * ~5 × 10 ** 41 250 0.1 M KOH Pd/FTO **** 42 * ~12 × 10−3 ** 42 * surface-normalized values (see information below); ** values estimated from the Tafel plots; *** glassy carbon; **** fluorine-doped tin oxide.

Furthermore, in order to fully access the suitability (and possibly also superiority) of the developed electrodes for their possible commercial use, Table 3 also presents the Rct and j (η = 0.3 V) parameter values for the OER, obtained on selected noble metals/oxide (Pt, RuO2 Catalysts 2021, 11, 468 and Pd), under similar experimental conditions [40–42]. Hence, the recorded (or estimated)9 of 13 charge-transfer resistance parameter values for bulk Pt, RuO2/GC and Pd/FTO electrodes were significantly greater than those recorded for the Co (or Mo)-modified nickel foam specimens. On the other hand, only fluorine-doped tin oxide palladium electrode exhibited radically electrode exhibited radically higher values (ca. 12 × 10−3 A cm−2) of Tafel plot-derived higher values (ca. 12 × 10−3 A cm−2) of Tafel plot-derived current-density parameter than those current-density parameter than those recorded in the present work for the catalyst (Co, recorded in the present work for the catalyst (Co, Mo)-modified nickel foam materials. Mo)-modified nickel foam materials. In addition, the exchange current-density values (j0) for the OER recorded for Co- and In addition, the exchange current-density values (j0) for the OER recorded for Co- and Mo-modified Ni foam samples were calculated based on the linear dependence of -log Rct Mo-modified Ni foam samples were calculated based on the linear dependence of -log vs. overpotential/ƞ, fulfilled by kinetically controlled reactions through employing the Rct vs. overpotential/η, fulfilled by kinetically controlled reactions through employing Butler-Volmer equation and the relation between the j0 and the Rct parameter for overpotential the Butler-Volmer equation and the relation between the j0 and the Rct parameter for overpotentialapproaching 0 approaching [43,44]. Hence, 0 [43 the,44 impedance-ba]. Hence, the impedance-basedsed exchange current-density exchange current-density value for the ƞ valuestudied for overpotential the studied overpotential range ( = 270–620 range (mVη = with 270–620 50 mV mV potential with 50 mV increments) potential came increments) to 6.06 −5 −2 came× 10 toA 6.06cm ×for10 the−5 Aas-received cm−2 for theNi foam as-received sample. Ni Ho foamwever, sample. significantly However, higher significantly values of the j0 parameter were recorded for both Co- and the Mo-modified Ni foam samples, i.e., 2.89 higher values of the j0 parameter were recorded for both Co- and the Mo-modified Ni foam −4 −2 −4 −2 samples,× 10 A cm i.e., 2.89and ×1.0410 ×− 410A cmA cm−2 and, correspondingly 1.04 × 10−4 A cm(see− 2Figure, correspondingly 6 below). In (seeaddition, Figure the6 Rct-derived exchange-current densities are somewhat higher in comparison to those of other below). In addition, the Rct-derived exchange-current densities are somewhat higher in comparisonOER works to performed those of other on OERelectrodes works modified performed by on Co electrodes and Mo modified elements by Coin andalkaline Mo elementssolutions in[25,45]. alkaline There, solutions the [25exchange,45]. There, current-density the exchange current-densityvalues recorded values for Ni recorded + Mo forcomposite Ni + Mo coating composite came coating to 9.7 came× 10−5 toA cm 9.7−×2 [25]10− and5 A cm5.2− ×2 10[25−5 ]A and cm 5.2−2 [45].× 10 Similarly,−5 A cm− values2 [45]. 0 Similarly,of the j valuesparameter, of the recordedj0 parameter, for recordedhigh overpotential for high overpotential range on Co-based range on Co-basedcompsite compsiteelectrodes electrodes came to 3.5 came × 10 to−5 3.5A cm× −102 [46]−5 A and cm 1.1−2 [×46 10]− and5 A cm 1.1−2× [28].10− 5 A cm−2 [28].

Figure 6. –log Rct vs. overpotential relationship for the OER on oxidized pure Ni foam, Co- and Figure 6. –log Rct vs. overpotential relationship for the OER on oxidized pure Ni foam, Co- and Mo-activated Ni foam electrode surfaces, carried-out in 0.1 M NaOH solution. Mo-activated Ni foam electrode surfaces, carried-out in 0.1 M NaOH solution.

3.3. Materials andand MethodsMethods 3.1.3.1. Solutions andand ChemicalChemical ReagentsReagents InIn thisthis work,work, allall supportingsupporting solutionssolutions werewere preparedprepared byby dissolvingdissolving chemicalchemical reagentsreagents inin ultra-pureultra-pure water water (18.2 (18.2 M MΩΩ cm)cm) from from the the Hydrolab Hydrolab (Sprin (Springg 30 30s) purification s) puriﬁcation system. system. 0.1 0.1M MNaOH NaOH working working electrolyte electrolyte was was prepared prepared fr fromom sodium sodium hydroxide pellets (99.996%,(99.996%, AESAR,AESAR, WardWard Hill,Hill, NY,NY, USA).USA). BeforeBefore eacheach seriesseriesof of experiments, experiments, atmosphericatmospheric airair waswas removedremoved from from central central and and both both side side cell’s cell’s comp compartmentsartments by bubbling by bubbling with withhigh purity high purity argon argon(Ar 6.0 (Ar grade, 6.0 grade,Linde, Pu Linde,ławy, Puławy, Poland) Poland) for 15 and for 5 15 min, and respectively. 5 min, respectively. Furthermore, Furthermore, argon gas argon gas ﬂow was kept above the solutions throughout the experiments. 0.5 M H SO flow was kept above the solutions throughout the experiments. 0.5 M H2SO4 (stock H22SO44: (stock H2SO4: 96%, Merck, Darmstadt, Germany) solution was used for periodic charging of a Pd reversible hydrogen electrode (RHE).

3.2. Preparation of Electrochemical Cell and Electrodes The OER activity of pure Ni foam, Co- and Mo-modiﬁed Ni foam electrodes was examined in an electrochemical cell made of Pyrex glass. The cell comprised three separate compartments. In the central compartment, a Ni foam-based working electrode (WE) was Catalysts 2021, 11, 468 10 of 13

placed, when both Pd (1.0 mm diameter wire, 99.98+% purity, AlfaAesar, Ward Hill, NY, USA) reversible hydrogen electrode and a Pt (1.0 mm diameter wire, 99.9998% purity, Johnson Matthey, Inc., London, UK) counter electrode (CE) were located in separate side compartments. Nickel foam base electrode material was delivered by MTI Corporation (Richmond, CA, USA) (purity: >99.99% Ni; thickness: 1.6 mm; surface density: 346 g m−2; porosity: ≥95%; estimated electrochemically active surface 14.9 cm2)[38]. All examined electrodes were 1 cm × 1 cm (ca. 35 mg). Before electrodeposition of Co and Mo elements on Ni foam base material, freshly cut foam samples were degreased in acetone bath for 15 min under ultrasonication. Then, they were air dried and etched in 2 M HCl for 15 min at 60 ◦C, also under ultrasonication. Electrodeposition of Co and Mo catalysts on Ni foam was performed according to the conditions and bath compositions shown in Table4. The bath’s pH for Co electrodeposition was readjusted with 0.1 M NaOH or 0.1 M HCl solutions prior to the experiment. Similarly, before electrodeposition of Mo, the solution’s pH was modiﬁed with 0.1 M NaOH, accordingly. No signiﬁcant pH change was observed during both metal deposition trials.

Table 4. Operating parameters and bath compositions employed for electrodeposition of Co and Mo onto Ni foam substrate.

Bath Constituents Concentration (M) Operating Parameters Electrodeposition of Co

CoCl2 × 6 H2O (99%, Sigma Aldrich, Saint Louis, MO, USA) 0.10 Anode: Pt foil Cathode: Ni foam Temperature: 303 K NaCl (99.9%, POCH, Gliwice, Poland) 0.02 Deposition time: 300 s Current-density: 0.52 mA cm−2 Solution pH: 4.5 Electrodeposition of Mo Anode: Pt foil Cathode: Ni foam (NH ) Mo O ·4H O(≥99.3%, Merck, Darmstadt, Germany) −4 4 6 7 24 2 1.95 × 10 Temperature: 303 K CH CO NH (≥98.0%, Merck, Darmstadt, Germany) 3 2 4 0.26 Deposition time: 1800 s CH COOK (≥99.0%, Sigma Aldrich, Saint Louis, USA) 3 0.26 Current density: 5.2 mA cm−2 Solution pH: 6.8

Such-modified Ni foam materials were left in a desiccator for an extended period of time before being weighed. Before conducting a series of electrochemical experiments, as received, Co- and Mo-modified Ni foam samples underwent extensive electrooxidation by means of 50 cyclic voltammetry sweeps, carried-out for the potential range 1.2–1.9 V/RHE, at a sweep rate of 50 mV s−1. The palladium RHE was sealed in soft glass. Before its use, the reference electrode was cleaned in hot, concentrated sulphuric acid, followed by cathodic (galvanostatic) charging with hydrogen in 0.5 M H2SO4 until H2 bubbles in the electrolyte were clearly visible. The stability of such prepared RHE was occasionally checked by recording its potential shift in time (it should then be noted that all potentials throughout this work are given on the RHE scale). The CE electrode, prior to its use, was flame-annealed. Similarly, before each series of electrochemical experiments, the electrochemical cell was taken apart and soaked in hot sulphuric acid for at least 4 h. After having been cooled to about 30 ◦C, the cell was thoroughly rinsed with Hydrolab ultra-pure water.

3.3. Experimental Methodology Electrochemical performance of the OER electrocatalysts (pure Ni foam, Co-, and Mo-modiﬁed Ni foam electrodes) was conducted by a.c. impedance spectroscopy and quasi steady-state Tafel polarization techniques by means of an AUTM204 + FRA32M Catalysts 2021, 11, 468 11 of 13

Multi-Autolab potentiostat/galvanostat system, at room temperature (293 K). For the a.c. impedance measurements, the generator provided an output signal of 5 mV rms and the frequency was swept between 1.0 × 105 and 2 × 10−2 Hz. The instruments were controlled by Nova 2.1 software for Windows (Metrohm Autolab B.V., Opacz-Kolonia, Poland). Also, all data analysis was performed with the above-mentioned software package. Typically, three impedance measurements were performed at each potential value, independently at two foam electrodes, where reproducibility of such obtained results was usually below 10%. The obtained impedance spectra were fitted by means of a complex, nonlinear, least- squares immittance fitting program. Furthermore, quasi-potentiostatic anodic polarization experiments (recorded at a scan-rate of 0.5 mV s−1) for the OER were carried-out at all Ni foam-based samples. Additionally, SEM/EDX surface spectroscopy characterization of all examined (Ni foam, Co- and Mo-modified Ni foam) samples was carried out by means of Merlin FE-SEM microscope (Zeiss, Jena, Germany), equipped with Bruker XFlash 5010 EDX instrumenta- tion (with 125 eV resolution) (Bruker, Billerica, MA, USA).

4. Conclusions Electrochemically deposited small amounts of transition metals in the form of cobalt (at ~1 wt.%) and molybdenum (at ~0.28 wt.%) nanoparticle deposits on the surface of Ni foam material significantly enhanced catalytic activity of the base foam material towards anodic evolution of oxygen in 0.1 M NaOH solution. The above is primarily related to superior OER catalytic properties of Co and Mo nano-deposits, in addition to the considerable modification of electrochemically-active surface for these catalyst materials. The results of this work showed a real possibility for replacing expensive and scarcely found in the natural environment noble metals with significantly cheaper transition metals for the mass production of commercial alkaline water electrolysers.

Author Contributions: Conceptualization, B.P.; methodology, B.P., M.Ł. and T.M.; investigation, M.Ł., M.K; data curation, T.M., M.L. and M.K.; writing—original draft preparation, M.Ł., T.M.; editing, B.P. and T.M.; supervision, I.M.K. All authors have read and agreed to the published version of the manuscript. Funding: This work was financed by the research grant of the National Science Centre, Miniatura 1 project no. 2017/01/X/ST4/00881; Project financially supported by the Minister of Science and Higher Education in the range of the program entitled “Regional Initiative of Excellence” for the years 2019–2022, Project No. 010/RID/2018/19, amount of funding 12.000.000 PLN. The results in this paper were obtained as part of comprehensive study financed by the University of Warmia and Mazury in Olsztyn, Faculty of Agriculture and Forestry, Department of Chemistry (grant No. 30.610.001-110). Acknowledgments: Mateusz Luba would also like to acknowledge a scholarship from the program Interdisciplinary Doctoral Studies in Bioeconomy (POWR.03.02.00-00-1034/16-00), funded by the European Social Fund. Conflicts of Interest: The authors declare no conflict of interest.

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